A molded coil

ABSTRACT

A mold transformer consisting of a blank coil impregnated with an electrical insulating resin composition of epoxy resin or the like, said blank coil having as an insulator a highly impregnable non-woven polyester fabric or non-woven glass fabric interposed between a high-voltage winding and a low-voltage winding, between the high voltage winding and a magnetic core and between the lowvoltage winding and the magnetic core respectively, and a nonwoven polyester fabric between adjacent winding layers of each of the high-voltage and low-voltage windings; and a method of producing said mold transformer.

United States Patent 191 Yamashita et al.

1 A MOLDED COIL [75] Inventors: Kazuo Yamashita; Harutoki Nakamura; Koichi Hirakawa, all of Toyonaka, Japan [73] Assignee: Northern Industries & Mfg., Inc.,

Saint Paul, Minn.

22 Filed: vJtn 12,1970

211 Appl. No.: 64,844

' Related u.s. Application Data [62] Division Of Ser. No. 778,715, Nov. 25, 1968, abancloned.

[52] US. Cl ..336/205, 336/206 [51] Int. Cl ..H01f 27/32 [58] Field of Search.....336/205, 206, 96; 174/121 R, 174/25 R; 317/258 [56] References Cited UNITED STATES PATENTS 2,935,668 5/1960 Robinson 3.317/258 [4 1 Jan. 16, 1973 2,993,949 7/1961 Moebius ..l74/l2l R 3,263,196 7/1966 Reber ..336/205 X 3,532,800 10/1970 Wyly ..l74/25 R Primary Examiner-E. A. Goldberg Attorney-Stevens, Davis, Miller & Mosher [57] ABSTRACT A mold transformer consisting of a blank coil impregnated with an electrical insulating resin composition of epoxy resin or the like, said blank coil having as an insulator a highly impregnable non-woven polyester fabric or non-woven glass fabric interposed between a high-voltage winding and a low-voltage winding, between the high voltage winding and a magnetic core and between the low-voltage winding and the magnetic core respectively, and a non-woven polyester fabric between adjacent winding layers of each of the high-voltage and low-voltage windings; and a method of producing said mold transformer.

7 Claims, 18 Drawing Figures MOLDED COIL This disclosure is a division of our U. S. Pat. application Ser. No. 778,715, filed Nov. 25, 1968, now abancloned.

This invention relates to a resin-molded dry type transformer and a method of producing the same.

In recent years, there has been a strong demand for electric equipment, particularly high-voltage equipment, which are small in size, light in weight and safe in operation. In the field of power transformers, the classification of electrical insulation has been elevated to class B insulation or class H insulation, and various studies have been made on a resin-molded dry type transformer by reason of its high reliability and easy maintenance and inspection.

A coil unit used in the resin-molded dry type transformer has heretofore been produced by producing a coil bobbin from a laminated plate of glass cloth, mica board, asbestos board or the like impregnated with silicon resin, forming a low-voltage winding on said coil bobbin and then forming a high-voltage winding on the outer peripheral surface of said low-voltage winding ,with an insulator layer of polyester film, glass cloth, mica paper and board, asbestos paper and board or the like interposed therebetween. ln forming the high-voltage and low-voltage windings, each winding layer has been insulatedfrom the adjacent layers by polyester film, asbestos paper or the like insulating material. The coil unit thus produced is placed in a suitable mold with the lead wires extending to the outside of said mold and impregnated therein with a suitable electrically insulating resin composition (hereinafter referred to as resin composition) under vacuum, whereby a mold coil having the high-voltage and the low-voltage windings enclosed and insulated by said resin composition is obtained, said resin composition being composed primarily, for example, of bisphenol-type epoxy resin to which a suitable plasticizer, hardening agent and hardening promotor, and if necessary an inorganic filler, such as SiO or A1 are added in suitable amounts. The mold coil produced in the manner described above poses no problem if the spacings between the component coils and between adjacent winding layers are completely impregnated with the resin composition, but according to the method described above, it has been quite difficult to thoroughly impregnate the mold coil with the resin composition. lf a voltage is applied to the mold coil under such condition, corona discharge takes place at portions where the resin composition is not completely impregnated, with the result that the equipment becomes unserviceable due to deterioration of said portionsoln addition, when the mold coil is not thoroughly impregnated with the resin composition, the heat generated in the coil cannot be dissipated satisfactorily, so that the mold coil becomes heated during use rendering itself incapable of performing a satisfactory function. ln order to eliminate such drawbacks, the so-called double casting method, that is, a method wherein a blank coil is impregnated, first with a resin composition of lower viscosity and then with a resin composition of higher viscosity, has been attempted frequently, but this method has not proved entirely satisfactory.

The present invention provides a method which obviates the aforesaid drawbacks of the conventional method and by which a material coil can be impregnated with a resin composition readily thoroughly, whereby a coil is obtained which operates safely over an extended period. Namely, the method of this invention is characterized by the use of a special material as an interlayer insulator and as an insulator between a high-voltage and a low-voltage winding for the purpose of improving the impregnability of the material coil, although the steps of producing a coil is not substantially different from those of the conventional method. The method of the invention is also characterized by the use of a resin composition which contains a special inorganic filler or fillers for the purpose of enhancing the heat dissipating property and the crack-resistance of the product mold coil.

For a fuller understanding of the nature and objects of this invention, reference should be made to the following detailed description taken in connection with the accompanying drawings wherein;

FIG. 1 is a perspective view, partially broken away, of a coil comprising an embodiment of the mold transformer according to this invention;

FIGS. 2a and 2b and FIGS. 30, 3b and 3c are views showing in section the structures of metallic frames used in the formation of a coil bobbin according the present invention and an embodiment of the coil bobbin forrned by the use of said metallic frame respectivey;

FIGS. 4a and 4b are sectional views of an interlayer insulation having the edges parallel to the winding direction thereof worked according to a conventional method;

FIGS. 5a and 5b and FIGS. 6, 7, 8 and 9 are sectional views of several forms of the interlayer insulation having the edges parallel to the winding direction thereof worked accoding to the method of this invention;

FIG. 10 is a plan view of an interlayer insulation hav ing the edges parallel to the winding direction thereof worked according to the method of this invention;

FIG. 11 is a sectional view of another form of the interlayer insulation having the edges parallel to the winding direction thereof worked according to the method of this invention;

FIG. 12 is a view showing one form of a resin guide tube used in the present invention; and

FIG. 13 is a diagram graphically showing the amount of burned MgO which can be mixed in an epoxy resin as a filler until the viscosity of said epoxy resin reaches 300 c.p.s., relative to the temperature at which said MgO is burned.

The method of this invention will be described stepby-step with reference to the accompanying drawings. A coil bobbin used in the present invention has not only large mechanical strength but also excellent heat stability and impregnability. Namely, as shown in FIG. 2a the coil bobbin comprises a metallic frame 7 and a thin layer 8 of readily impregnable insulating material, e.g., a layer of non-woven polyester or non-woven glass fabric, formed around the outer peripheral surface of said metallic frame in a thickness to provide a sufficient insulation. The term non-woven glass fabric as used herein means chopped glass strands bound together in the shape of a fabric with a heat-stable resin, such as polyester or epoxy resin. After a coil has been formed, a slit 7' is formed in the metallic frame 7 extending over the entire length thereof as shown in FIG. 2b, so that said metallic frame will not form a closed electric circuit. Alternatively, the slit 7' may be formed in the metallic frame 7 before the formation of the coil as shown in FIG. 3. In this case, a strip 9 of electrically insulating material is disposed in the slit 7' so that the metallic frame 7 may not form a closed electric circuit but yet maintain a required mechanical strength, and then the aforesaid thin layer of insulating material is formed around the outer peripheral surface of said metallic frame to form a coil bobbin. The dielectric strength of the coil bobbin may be further increased by the use, in place of the readily impregnable insulating material mentioned above, of an insulating material which has previously been impregnated with a semicured resin, e.g., a semi-cured epoxy resin. The metallic frame is left to remain in the resultant mold coil as a part thereof, but if desired, it may be removed after formation of a coil thereon and before the coil is impregnated with a resin composition, or it may be used as a portion of a mold during the process of impregnating the coil with a resin composition and removed after said resin composition has been cured. When the metallic frame is to be removed, it will be obvious that no consideration is required to avoid the formation of a closed electric circuit but the metallic frame may be of such a structure which facilitates the removal of said metallic frame.

After the coil bobbin is formed in the manner described above, an electromagnetic wire is wound around said coil bobbin to form a high-voltage (or lowvoltage) winding. In the formation of the coil, an insulation layer having a desired dielectric strength is interposed between adjacent layers of the winding. The present invention is characterized by the use of a nonwoven polyester fabric as the interlayer insulation. The non-woven polyester fabric used in the present invention is characterized in that not only it is excellent in thermal, mechanical and electric properties but also it is more readily impregnable with a resin composition (because it is lower in density) and less expensive than that used heretofore. In the formation of a coil, the turns at the starting portion and the terminal portion of each layer tend to be displaced relative to the turns in the layer and in the worst case the turns fall into the inner layer of the winding. In order to eliminate such trouble, it has'been commonly practiced to fold the edges of an interlayer insulation 10, extending parallel to the winding direction, as indicated by 11 in FIG. 4a.

Such a method may be effective when the interlayer insulation used consists of such a material as kraft paper having high bending strength but is not effective for materials of relatively high flexibility, such as the nonwoven polyester fabric used in the present invention because the materials hardly remain in the folded state. Further, had the edge of the interlayer insulation of such material be successfully folded, the folded edge would be readily strentched as at 12 in FIG. 4b when .subjected to even a slight force during the winding operation. The present inventors have conducted experiments using a non-woven polyester fabric and found that when the edges of the non-woven polyester fabric, extending in a direction parallel to the winding direction, are worked by any one of the methods to be described hereunder, a satisfactory result would be obtained with no displacement nor falling of the turns in a winding layer.

One of these methods is illustrated in FIGS. 5a and 5b. Namely, according to this method, an electric insulating material 13 having a suitable thickness is wrapped by the edge portion of the insulating fabric in such a manner that the height of the bank thus formed becomes substantially equal to the diameter of an electromagnetic wire. According to another method illustrated in FIG. 6, an electric insulating material 14 of a suitable thickness is placed along the edge of the insulating polyester fabric and melt-bonded thereto with heat under pressure. FIG. 7 shows still another method in which the edge portions of the non-woven polyester fabric are folded over several times and the resultant folds are melt-bonded together by heating the same under pressure. According to stillanother method illus trated in FIG. 8, the edge portions are folded once or several times and the resultant folds are sewed together by a thread 15 of nylon, polyester, etc. According to still another method illustrated in FIGS. 9a and 9b, the edges of the fabric is folded with a nylon or polyester thread 16 of substantially the same diameter as that of the electromagnetic wire enclosed therein and both the upper and lower sections of the folded edge are sewed together by a nylon or polyester thread at the center thereof or at the edge of said upper section. FIG. 10 shows still another method wherein stitches of a nylon or polyester thread are formed along each edge portion of the fabric by sewing. FIG. 11 shows still another method wherein the edge portions of the fabric are folded once or several times and after impregnating the folded edges with a liquid or powdery composition consisting of an epoxy resin and a hardening agent by immersing or spray method, the epoxy resin is semi-cured, whereby the folds are fixed as at 17. This method may be used in combination with the methods illustrated in F IGS. 5 to 10.

Besides the methods exemplified above, other methods may also be employed for working the edges of a non-woven polyester fabric, extending parallel to the winding direction. It is also to be understood that although in the foregoing description the various methods are explained as applied to a non-woven polyester fabric, these methods can similarly be effectively operated with interlayer insulations of other materials which can be used in place of the non-woven polyester fabric.

After the low-voltage (or high-voltage) winding has been formed in the manner described, an insulation layer having a thickness sufficient for the insulation between high-voltage and low-voltage windings is formed on said lowvoltage (or high-voltage) winding using the same material as that used for the formation of the coil bobbin, e.g., non-woven polyester fabric or non-woven glass fabric, as by wrapping. In this case also, a thin insulating material impregnated with a semi-cured resin may be effectively used. Thereafter, a high-voltage (or low-voltage) winding is formed on the insulation layer while interposing a non-woven polyester fabric, having the edges thereof worked in the manner described above, between adjacent layers of winding.

Upon completion of the formation of the high-voltage (or low-voltage) winding, the resultant assembly is insulated with the non-woven polyester fabric, nonwoven glass fabric or semi-cured resin-impregnated thin insulating material as described above, and thus a blank or non-impregnated coil is obtained.

The blank coil thus obtained is placed in a suitable mold and after drying sufficiently, impregnated with a resin composition under vacuum of l to 3 mmHg. According to the present invention, the coil can be impregnated with the resin composition readily completely throughout the interior thereof, since a low density material, such as non-woven polyester fabric or nonwoven glass fabric, is used as an insulating material as stated hereinabove. Although the coil can be sufficiently impregnated with the resin composition by the method described above, the velocity of impregnation may be further increased by interposing a hollow tube between the low-voltage and high-voltage windings or between adjacent winding layers of each of the windings, said hollow tube being made of an electrically insulating material and having apertures perforated in the wall thereof, as shown in FIG. 12, providing for the passage of a resin composition therethrough. Thereafter, the resin composition is cured with heat and the mold is removed to obtain a mold coil. The mold coil thus produced is assempled with an iron core and other component parts to form a transformer.

For reducing the size of the resin mold transformer, it is necessary to form radiating passages in the coil or to provide radiating plates on the coil surface, but it is also one of the important means to improve the thermal conductivity of the resin composition used.

For this purpose, it has been practiced to add to the resin composition an inorganic filler having high thermal conductivity. However, addition of the filler has' the disadvantage that the viscosity of the resin composition becomes higher as the amount of the filler added increases, and accordingly impregnation of the coil with the resin composition becomes difficult. It has, therefore, been one of the important problems to find a way of increasing the amount of the filler added while maintaining the viscosity of the resin composition at a low level. This problem has been solved by the present invention. Namely, the present invention is characterized by the use of MgO, which has been burned at elevated temperatures, as part of the filler. The chart of FIG. 13 shows the amount by part of burned MgO which can be added to I00 parts of epoxy resin before the viscosity of the resultant resin composition reaches 300 c.p.s., relative to the temperature at which MgO is burned. As will be seen in the chart, the amount of burned MgO which can be added to the resin increases sharply as the burning temperature exceeds l0O0C. That is to say that MgO burned at temperatures higher than l0O0C. can be added in large amounts to the resin without substantially increasing the viscosity of the resultant resin composition. The experiment has revealed that in comparing a resin composition comprising MgO which has been subjected to a heat treatment at l200C. with a resin composition of the same viscosity comprising MgO which has not been subjected to the heat treatment, the thermal conductivity of the former is about eight times as high as that of the latter on curing. In practice, the filler which has been subjected to a heat treatment is used in a mixture of other fillers, such as SiO Alternatively, MgO may be mixed with other fillers, such as SiO first and then subjected to a heat treatment at l0O0C. or higher, to obtain substantially the same result with sufficient mechanical strength.

As stated above, a resin composition having high thermal conductivity can be obtained by using a filler consisting of or consisting primarily of MgO having previously been subjected to a heat treatment, and by the use of such a resin composition it is possible to reduce the size of the product mold transformer.

According to the present invention, therefore, a mold transformer can be obtained which is small in size and excellent in performance. However, what should be borne in mind in producing the mold transformer is the fact that cracks tend to occur in the surface of the mold coil due to curing shrinkage or the like of the resin composition in the process of curing of said resin composition by heat. This tendency does not substantially appear when use is made of a resin composition which is relatively low in thermal deflection temperature but appears frequently when use is made of a resin composition which is relatively high in thermal deflection temperature. This problem has also been overcome by the present invention. Namely, according to the method of this invention the blank coil is impregnated with a resin composition in a mold, with the spacing between said coil and said mold filled with chopped strands of glass or synthetic resin having a length from 1 to 20 mm. In this way, it is possible not only to completely avoid occurrence of cracks due to curing shrinkage of the resin composition but also to increase the resistance to thermal cycle of the mold coil. The length of the chopped strands is specified to be from 1 to 20 mm. because use of a fiber shorter than 1 mm. will result in a reduced strength of the cured resin composition against a mechanical stress, whereas use of a fiber longer than 20 mm. will result in orientation of the fiber with the result that the cured resin composition is resistive to a stress in one direction but not resistive to stresses in the other directions. Namely, the effect of the fiber on the curing shrinkage of the resin composition, wherein said resin composition is subjected to mechanical stresses in all directions, is diminished.

As will be understood from the foregoing description, it is possible according to the invention to produce a resin mold transformer having excellent electrical, thermal and mechanical properties.

Now, the present invention will be further illustrated by way of example. A material coil was produced using as an insulator a non-woven glass fabric between the high-voltage winding and the low-voltage winding between the high-voltage winding and the magnetic core and between the low-voltage winding and the magnetic core, and a non-woven polyester fabric of the type shown in FIG. 8 between adjacent layers of each coil winding. The blank coil thus produced was placed in a mold and after filling the space thcrebetween with chopped strands of glass having a length of 3 mm., the coil was impregnated with a resin composition under vacuum of l to 3 mmHg. Using this coil, a transformer was produced. Upon comparing the transformer with a mold transformer produced by a conventional method, it was found that the corona voltage of the former was about percent higher than that of the latter and the dielectric strength of the former was about 50 percent greater than that of the latter. Further, the mold transformers produced according to the method of this invention showed no substantial variation from one another in electrical and thermal properties.

In a mold transformer, the materials interior thereof are subjected to less oxidation and accordingly deteriorated at much slower speed than the material in direct contact with air. This makes it possible to use an insulating coating of lower classification of electrical insulation on the coil winding to be molded in the resin composition, it being only necessary to use an insulating coating of higher classification of electrical insulation on lead wires which are connected to the terminal ends of said coil winding within the resin composition and extending to the outside in direct contact with air. This is advantageous in reducing the cost of the mold transformer. It is believed that the method of this invention by which a mold transformer having excellent electrical properties can be produced at low costs as described above, is of great industrial value.

What is claimed is:

l. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a lowvoltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic coreas insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the extended ends of the interlayer insulators lying in parallel with the winding direction is folded with an electrically insulating material of a desired thickness disposed therewithin so that the folded end may have a height substantially equal to the diameter of a wire of the coil to thereby prevent slipping out of the wound wires.

2. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a lowvoltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the interlayer insulators has a strip of an insulating material of a desired thickness placed on each of its extended ends lying in parallel with the winding direction, said insulators and insulating material being heated and pressed to be fused together.

3. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a lowvoltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the interlayer insulators has its extended ends lying in parallel with the winding direction folded at least once andeach of the folded portions is sewed together by a thread of nylon, polyester or the like material.

4. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a lowvoltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a .blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the interlayer insulators has its extended ends lying in parallel with the winding direction folded at least once with a thread of nylon, polyester or the like material disposed therewithin, each of the folded portions being sewed together by a thread of nylon, polyester or the-like material;

5. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a lowvoltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the interlayer insulators has its extended ends lying in parallel with the winding direction folded once or several times, each of the folded portions being applied with a liquid or a powder by an immersing or spray method which liquid or powder comprises an epoxy resin and a hardening agent heated to obtain a semi-cured state (a state of B- stage) of said epoxy resin.

6. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a lowvoltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and nonwoven polyester fabrics interposed between adjacent. layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such} as "an epoxy resin, wherein each of the interlayer insulators has its extended ends lying in parallel with the winding direction folded once or several times, and each of the folded portions is sewed together by a thread of nylon, polyester or the like material, applied with a liquid or'a powder by an immersing or spray method which liquid or powder comprises an epoxy resin and a hardening agent heated to obtain a semi-cured state (,a state of B- 0 stage) ofsaid epoxy resin. t

7. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a lowvoltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of has each of its extended ends lying in parallel with the winding direction stitched with a thread having a diameter and strength sufficient to prevent slipping out of the wound wires. 

2. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a low-voltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the interlayer insulators has a strip of an insulating material of a desired thickness placed on each of its extended ends lying in parallel with the winding direction, said insulators and insulating material being heated and pressed to be fused together.
 3. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a low-voltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the interlayer insulators has its extended ends lying in parallel with the winding direction folded at least once and each of the folded portions is sewed together by a thread of nylon, polyester or the like material.
 4. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a low-voltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the interlayer insulators has its extended ends lying in parallel with the winding direction folded at least once with a thread of nylon, polyester or the like material disposed therewithin, each of the folded portions being sewed together by a thread of nylon, polyester or the like material.
 5. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a low-voltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the interlayer insulators has its extended ends lying in parallel with the winding direction folded once or several times, each of the folded portions being applied with a liquid or a powder by an immersing or spray method which liquid or powder comprises an epoxy resin and a hardening agent heated to obtain a semi-cured state (a state of B-stage) of said epoxy resin.
 6. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a low-voltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators tO form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the interlayer insulators has its extended ends lying in parallel with the winding direction folded once or several times, and each of the folded portions is sewed together by a thread of nylon, polyester or the like material, applied with a liquid or a powder by an immersing or spray method which liquid or powder comprises an epoxy resin and a hardening agent heated to obtain a semi-cured state (a state of B-stage) of said epoxy resin.
 7. A molded transformer comprising a coil having a highly impregnable, heat-stable non-woven fabric interposed between a high-voltage winding and a low-voltage winding, between the high-voltage winding and a magnetic core and between the low-voltage winding and the magnetic core as insulators and non-woven polyester fabrics interposed between adjacent layers of the windings of both the high-voltage winding and the low-voltage winding as interlayer insulators to form a blank coil, said blank coil being impregnated with an electrically insulating resin composition such as an epoxy resin, wherein each of the interlayer insulators has each of its extended ends lying in parallel with the winding direction stitched with a thread having a diameter and strength sufficient to prevent slipping out of the wound wires. 